The present invention concerns a method of charging sealed nickel-cadmium Ni-Cd secondary cells, normally assembled in batteries. The invention particularly concerns a method of charging batteries for use as an energy source for ground vehicles or aircraft.
Ni-Cd batteries which are in current use require frequent maintenance. Replacing them with batteries which do not require maintenance would therefore be of interest. However, such batteries must be interchangeable with traditional batteries, in particular as regards their performance on rapid discharge at low temperature and their required cycling speed.
In aviation, these batteries are used in two different ways. Firstly, they are used to start turbines, which can occur several times during a flight, with small discharge depths and at temperatures in the range -40.degree. C. to +60.degree. C. Secondly, they act as a safety measure in the case of a malfunction in the power for an airplane, where discharge depths may be as high as 100% at temperatures in the range -20.degree. C. to +60.degree. C.
On the ground, an electric vehicle must be usable at any time.
These batteries must therefore be permanently capable of carrying out their function, and as a consequence they must always be at least partially charged. Users thus require that batteries are immediately recharged, despite their high temperature, immediately after discharge. In other cases, when batteries are not used for long periods, they are in a partially discharged state and their temperature is that of the surroundings, which can sometimes by very low. Nevertheless, the batteries must be capable of being recharged quickly and efficiently. In addition, in some cases, the time allowed for recharging does not exceed 60 minutes (min).
Complete high rate charging of a Ni-Cd cell is carried out in two stages. In the first stage, the active material in the electrodes is oxidoreduced. When all the active material in the positive electrode has been transformed, the cell passes into an overcharging stage. During this second stage, oxygen is evolved at the positive electrode. Recombination of the oxygen at the negative electrode results in a rise in temperature which has the secondary effect of reducing the voltage in the cell.
Normally, in order to ensure that the cell is fully charged, change in voltage or temperature is monitored and charging is halted after a fixed period once voltage change becomes negative or when the temperature increases. In both these cases, the temperature of the cell is much higher at the end of charging than it was at the beginning. If the battery has to be used within a short period, the temperature has no time to drop. During the following recharge, the temperature of the battery, already very high at the start of charging, increases again. A rapid succession of charging and discharging soon results in a breakdown in the battery, since charging efficiency becomes poor and the charging rate drops with each recharge.
In a sealed cell, with a limited quantity of electrolyte, the presence of air in the cell acts as a thermal insulator and hinders heat evacuation. For vehicle batteries, these problems are aggravated still further for two reasons. The first is based on the fact that these batteries have a high capacity, of more than 10 Ah, and thus the thermal energy they generate is high. The second reason is that cell design does not favor heat evacuation. The container for the cells is formed from plastics material to limit battery weight, and it is a poor heat exchanger. In order to save space, prismatic cells are disposed side-by-side when mounted in a battery, which is particularly unfavorable for heat evacuation.